Optical coherence tomography (“OCT”), which is based on low coherence interferometry, measures the depth-resolved back-reflections or back-scattering from an object, for example a biological tissue. OCT can provide up to sub-micron-level image resolution, and at or above video-rate image capturing speed. OCT has demonstrated substantial potential as a minimally-invasive medical imaging modality. Early-stage OCT techniques alter the optical path length in the reference arm of an interferometer to introduce an optical group delay and records the interferogram time sequentially, which is commonly referred to as the time domain OCT (“TD-OCT”).
Recently, a group of alternative OCT approaches has attracted considerable attention since they have demonstrated extremely high imaging speed without any mechanical movement in the reference arm that drastically alleviates the complexity of scan mechanics used in TD-OCT. This group of approaches can be generally categorized as spectral domain OCT (“SD-OCT”) as these techniques all record the spectral interferograms that can be converted to depth-resolved backreflections by Fourier transform. Although spectral interferometry dates back to the original work of Michelson, Fourier transform approaches in terms of SD-OCT have only recently been applied to OCT. SD-OCT is generally divided into two general techniques: swept source OCT (“SS-OCT”); and Fourier domain OCT (“FD-OCT”), or spectral radar. The light sources also differ with different OCT operational modes. TD-OCT and FD-OCT use a wideband source, whereas SS-OCT usually utilizes a swept or tunable laser source.
For the calibration of SD-OCT signals, a variety of methods have been explored. Some of the most recognizable methods include using a fixed filter to pick up a specific wavelength as a point reference. This method can dynamically compensate the instability of the starting point of the sweeping but requires high repeatability of the spectrum. Another method includes using a Fabry-Perot (“FP”) interferometer or etalon to generate a frequency, f, comb function or a wavenumber, k, comb function. That is, while the laser wavelength is sweeping, the generated frequency or wavenumber comb function is a series of pulses with a fixed interval between adjacent two pulses. Similar to the FP method, a Mach-Zehnder interferometer (“MZI”) can be used to generate a frequency comb function, which may also be referred to as a frequency clock. Unlike the FP clock, the MZI clock is a sinusoid type fringe. This means that the crossing points, which are those points where the fringe signal crosses zero or any non-zero DC level, can also be determined, which provides twice the reference points than a FP clock with the same free spectral range (“FSR”). Balanced detection techniques are not easily implemented in the FP method, which leads to more phase errors in the calibration signal, as well as the potential for excess noise.
Many sophisticated swept laser sources provide a non-linear wavenumber-time (“k-t”) relationship, which suggests that a digitized OCT signal, while commonly sampled uniformly in time, is non-uniformly distributed in wavenumber space, or k-space. Poor or imprecise calibration could significantly degrade the system performance in terms of the resolution and the ranging accuracy, as well as other parameters. One study reported a hardware-based calibration by clocking the A/D converter with an uneven sampling in time to compensate for the non-linear sweeping operation. This can reduce the time consumption of software calibration, but increases the overall cost as the electronics are more complex, and is not feasible for different operation frequencies of the source. Similarly, a broadband source can be used and the frequencies swept, such as with a movable grating or prism. Other approaches may employ a wideband source with a tunable filter that scans or selects individual wavelengths. However, the power of any selected wavelength component is always much lower than the total power, which is a drawback of this approach. Moreover, a complex tuning/scanning mechanism is required for precise and repeatable functioning. To overcome the low-power limitation with previous approaches, the more common approach is to use a wideband source that is placed in an external cavity tunable laser as a gain medium, and in which a grating or prism is used as a tuning mechanism. Even in this more common approach, however, the issue of a complex tuning mechanism is present as a drawback.
A nearest neighbor check algorithm is popularly used in current SS-OCT systems for calibration. Its basic concept is using a sliding window with fixed width (e.g., 3 points or 5 points) to select consecutive subsets in the digitized clock data set, then searching for extrema in this subset as a final finding. This algorithm needs less computation and is presumably fast for calculation. However, its accuracy is substantially compromised as it is intrinsically sensitive to noise or phase errors in the calibration signal. Practically, prominent noise cannot be completely eliminated in the calibration signal, which may substantially affect the calibration accuracy. In addition, advanced calibration typically results in an increased processing burden.
Analog-to-digital (“A/D”) conversion is a signal-to-noise ratio performance limiting step in current systems employed for both SS-OCT and FD-OCT. The basic component to all A/D conversion is the quantizer, whose output is always the closest discrete level to the analog input. The interval between the discrete levels is usually uniform, which determines the quantization noise. Thus, the maximum SNR of a given A/D converter is limited by the total of these discrete reference levels. Any input signal with a higher SNR than that of the converter will inevitably suffer a signal loss in the stage of A/D conversion. The detection parameters of SS-OCT and FD-OCT appear to be inferior to TD-OCT, contrary to many current opinions. This concern may not be a practical issue for OCT imaging in transparent materials such as human eyes or certain plastics, which have low dynamic range. However, the concern could limit the capability of these techniques to penetrate many non-transparent materials, semi-transparent materials, non-biological materials, or layered or thick samples thereof without improved performance characteristics, including a larger dynamic range and improved SNR.
In light of the foregoing, it would therefore be desirable to provide a more effective SS-OCT calibration system and method that compensate for nonlinearities resulting from vector space conversion and non-repeatability and instability in source sweeping. In doing so, it would be desirable to provide such a method that is more accurate and reliable than those previously existing methods. It would also be desirable to provide a system and method for spectral domain OCT that exhibits a larger dynamic range and improved SNR.